Electrical contact



July 6, 1965 R. MANN ETAL ELECTRICAL CONTACT Filed sept. zo. 1951 INVENTORS ROBERT L. MANN DQ/VALD Hf. lli/R77 73,; i AGE United States Patent O 3,193,785 ELECTRCAL CNTAQT Robert L Mann, lilinneapolis, Donald W. Martin, St. Paul, Minn., assignors to Sperry Rand Corporation, New York, NX., a corporation of Delaware Filed Sept. Ztl, 1961, Ser. No. 139,47@ '7 Claims. (Cl. 33o-it The present invention relates generally to a variable iml pedance device for use in electrical circuits, and more specifically to an improved contact arrangement for use with such a device.

Adjustable impedances such as rheostats and variable inductors are often formed by winding an electrical conductor in spiral or helical fashion upon a cylindrical coil form. The Cassutt Patent 2,738,471, issued March 13, 1956 illustrates atypical arrangement. in this patent there is shown a cylindrical coil form on which is wound an elongated conductor. The coil form is mounted for rotation about its longitudinal axis and there is provided a pulley shaped electrical contact adapted to engaging a single turn on the winding. The axis of rotation of this pulley shaped Contact is substantially parallel to the axis of rotation of the cylindrical coil form. For reasons which will be explained, this type of contact is not wholly suitable in devices where it is desirable to lower the contact member so as to engage it with the conductor. One reason for this is that it is possible that the Contact may come down between adjacent turns to thereby ride along the coil form itself rather than along the conductor.

Another form of Contact often found in vairable impedance devices is the spring type wiper which is arranged to ride back and forth upon the surface of the closely spaced turns. rfhis type of arrangement has the inherent disadvantage that it is subject to localized wear, especially when the impedance element is designed to rotate. Because they are less subject to friction and resulting wear, the aforementioned pulley shaped contacts have largely superseded wiper arms in variable impedance devices. Pulley wheel contacts, however, have another drawback in that they are peculiarly susceptible to jamming because of eccentricities which may be present in the coil.

The present invention obviates these difiiculties by providing an electrical Contact which is less subject to localized wear than the prior art wiper contacts and which is relatively free from jamming and mislocating which is characteristic of the pulley shaped contact. More specifically, the Contact of the present invention is gear shaped, having teeth designed to be positioned between turns of the coil. The gear Contact is mounted for rotataion in the same manner that a gear is rotated by a worm. Because of the dimensioning of the turns of the coil and of the contact, a tooth of the gear lits between two adjacent turns. Therefore, when the contact is brought into engagement with the coil there is a positive locking or intermeshing of it with the gear. Also, because the contact is rotatably mounted, a substantially larger contact surface is provided, which distributes the wear over a large area and decreases the probability of failure due to wear.

It is accordingly a primary object of the present invention to provide an improved electrical contact for use with variable impedance devices.

Another object of this invention is to provide a rotatable impedance coil having an improved contact wheel cooperating therewith.

Still another object of this invention is to provide a selfpositioning contact operating in the same manner as a gear in a worm-andgear configuration and embodying the features of low wear due to a large surface area and a contact wiping action for low resistance.

The features of the invention which are believed to be novel are set forth with particularity in the appended ICC claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof may best be understood by reference tothe following description taken in connection with the acornmpanying drawing in which:

FIG. l illustrates a front view of the adjustable impedance and the manner in which the improved contact of this invention cooperates therewith; and,

FlG. 2 is a sectional view of the apparatus of FIG. l.

Referring to the drawing, the slide wire variable impedance, which may be either a rheostat or a variable inductor, illustrated comprises a frame including a pair of end plates 10 and l2 which are maintained in a spaced and parallel relationship by means of a pair of rods 14 and 16. The spacer rods may be fastened to the end plates by any one of several well known means. It is preferable that the end plates be formed from a suitable insulating material and that the rod 14 be made from a conductive material.

A coil form 1S comprising a hollow cylinder of a suitable insulating material is mounted on a shaft 20. The shaft is designed to fit into a pair of bearings 22 and 24 secured to the end plates 10 and 12, respectively, so that the coil form is free to rotate. The shaft 2t) extends through the bearing member 24 and the end plate 12 and a pinion Zo is aiixed thereto. The pinion meshes with a gear 2% which, in turn, is driven by a motor 3l). It should be understood that many other arrangements are available for effecting a rotataion of the coil form 18 and that the apparatus shown in FIG. l is merely illustrative.

The coil form 18 preferably has a spiral groove 32 of a suitable pitch cut on its outer cylindrical surface. This groove is designed to maintain the proper spacing between the turns of the conductive coil 34 which is placed in this helical groove. If the variable impedance device is a rheostat, the conductor 34 is preferably made from a more electrically resistive type of material than would be used when making a radio frequency inductor. However, when making a variable radio frequency inductorthe conductor 34 may be made from hard-drawn copper wire which is silver plated and coated with a tarnish resisting lubricant.

In order to make an electrical connection from the external circuit to the coil 34, a slip ring and brush arrangement may conveniently be employed. As is illustrated in FIG. l the left-hand end of the coil is connected to a strap 36 by means of a rivet 38. rthe strap leads to a band of conductive material 40 which girds the cylindrical coil form. While the band 4i) is shown as passing around the outside surface of the cylinder, it is of course possible to locate it on the inner surface or on the end of coil form. A wiping contact l2 cooperates with the slip ring lll) to provide a conductive path from the coil to an external electrical circuit (not shown).

Mounted on the support rod 14 is a contact carriage 44. As shown in FG. 2, the carriage may comprise a cylindrical band of conductive material which is wrapped around the support rod 14 so that it may be slid in a direction parallel to the longitudinal or rotational axis of the coil form 18. Spot welded or otherwise affixed to the carriage member 44 at the point 46 is' a strap of springlike conductive material 48. Aportion of this strap is bent downward in the neighborhood of the point 50 and in this depending portion a hole is provided. This hole may be threaded so that the mounting screw 52 may be affixed thereto. The screw 52 passes through the con` tact element 54, holding it to the spring member 4S. The area of the screw 52 where it passes through the contact 54 is left threadless and preferably is provided with a bearing surface for permitting a more friction-free rotation of the contact mounted thereon. Also, to prevent area-rse q l the rot-ation of the Contact from loosening this screw it may be desirable to provide a lock nut on it.

As can be most clearly seen in FIG. l, the contact member S4 is made from a circular piece of conductive material and has a plurality of notches regularly spaced about its periphery. The distance between adjacent notches from center to center is designed to correspond to the distance between adjacent turns of the coil 3ft. As such, the contact 54 has the general appearance of a worm gear while the coil 34 operates as a worm to drive this gear. The working depth of the notches on the contact member 54 is preferably slightly less than the radius ot the conductor used in the gear 3d so that the teeth do not ride upon the coil form T3. T his insures that good electrical contact will be maintained between the surface of the coil 3ft and the contact 5ft.

As the inductor is rotated by the motor 3d, the contact 5d will be slowly rotated, being driven by the turns of the coil in the same manner that a worm gear is rotated by the worm. Because the contact 5d rotates while it is in an operating position with respect to the coil, it offers a relatively large contact area so that contact wear is distributed. This is an inherent advantage the present invention over the conventional wiper arm contacts employed in prior art variable impedance devices.

In order to vary the impedance offered to an external electrical circuit (not shown) the carriage member liftmay be slid along the conductive support rod ltd so that a different number of turns is introduced into the external circuit. As the carriage is moved, the contact 54 walks over the turns of the coil. The contact of the present invention is also well suited for applications where it is desirable to raise and lower the contact so as to disengage and engage it with the coil 3d. ln prior art devices When the contact is lowered into engagement with the coil, it is possible that the contact member will fall between adjacent turns rather than directly on top of a single turn thereby making the electrical connection intermittent at best. Because a tooth on the contact of the present invention is designed to set between adjacent turns, the contact will mesh with the coil no matter what position the coil may be in at the time that the contact is brought down into engagement with it.

A further advantage inherent in the contact design of the present invention is that the contact has a much lower rotational speed than other rotating contacts. This is due to the fact that the coil must rotate one full revolution for the contact 54 to advance a distance of one tooth and notch. Wearing of the contact bearings is thereby reduced, increasing the reliability of the apparatus.

Various other modifications of the invention will suggest themselves to those skilled in the art. For example, it is possible to mount the conductive coil 34, the support shaft 14 and the contact carriage arrangement 44 and other necessary parts within the coil form l rather than outside thereof where the particular application makes this arrangement advantageous. It is to be understood therefore that the foregoing is to be interpreted as illustrative and not in a limiting sense except as required by the prior art and by the scope of the appended claims.

What is claimed is:

1. In a continuously variable electrical impedance device ofthe type having an electrical conductor wound as a helix upon a cylindrical coil form, said form being arranged for rotation about a longitudinal axis, an electrical contact for engaging said conductor comprising an electrically conductive disc member having a plurality of teeth spaced around its periphery, the spacing being such that at least one of said teeth meshes between adjacent turns ofthe conductor.

2. ln combination with a continuously variable electrical impedance device of the type having an electrical conductor wound as a helix upon a cylindrical coi form, a disc-shaped contact member having a plurality of notches spaced along the periphery thereof, the distance between adjacent notches being approximately equal to the spacing between adjacent turns on said coil form, and means for rotatably mounting said contact in engagement with said electrical conductor.

3. A contactor for use with a variable impedance of the type comprised of a substantially cylindrical insulating form mounted for rotation having a bare helical electrical conductor wound thereon comprising: a toroidally shaped piece of electrically conductive material having a plurality of regularly spaced notches located about its periphery having a working depth very slightly less than the radius of said helical conductor, the helical conductor cooperating with the notches of the toroidal piece to etect rotation thereof when said form is rotated.

d. In an adjustable electrical impedance element, the combination comprising: a cylindrical form mounted for rotation about a longitudinal axis; an electrically conductive wire wound as a helix on said form with a separation between adjacent turns; a rod xedly mounted external to said form and parallel to said longitudinal axis; a resilient member depending from said rod; a spur gear contact rotatably lixed to said resilient member having an axis of rotation substantially transverse to said longitudinal axis and having one tooth meshing between adjacent turns of said wire.

5. A variable impedance device comprising a substantially cylindrical insulating form and a bare helical electrically conductive coil wound thereon, means supporting said form for rotation about a longitudinal axis; a metal rod spaced from said form and positioned parallel to said longitudinal axis thereof; and a moving contacter carriage comprising a frame slidably engaging said rod, a spring member attached to said frame and a spur gear contact member rotatably mounted on said spring member, said gear contact member having an axis of rotation substantially normal to the longitudinal axis of said form, disposed so that the biasing force of said spring urges the spur gear contact member into engagement with the turns of said coil.

6. A variable impedance device comprising: an axially supported insulating form and a bare helical electrically conductive coil wound thereon; an electrically conductive rod spaced from said form and parallel to the lengthwise dimension thereof; a Contact or carriage compris- 'ag a frame slidably engaging said rod, a spring member attached to said frame and a spur gear contact rotatably mounted on said spring member on an axis of rotation normal to the axis of said form; and means for bringing said contact member into and out of engagement with said coil.

7. Apparatus as in claim 6 wherein said coil, said rod and said carriage are contained within said insulating form.

Retcrences Cited by the Examiner UNITED STATES PATENTS 926,925 5/09 Darlington 336-149 X 1,684,013 9/28 De Pue 338-145 X 2,480,340 8/49 Rose 336-139 FOREIGN PATENTS 237,683 8/25 Great Britain.

JOHN F. BURNS, Primary Examiner. 

1. IN A CONTINUOUSLY VARIABLE ELECTRICAL IMPEDANCE DEVICE OF THE TYPE HAVING AN ELECTRICAL CONDUCTOR WOUND AS A HELIX UPON A CYLINDRICAL COIL FORM, SAID FORM BEING ARRANGED FOR ROTATION ABOUT A LONGITUDINAL AXIS, AN ELECTRICAL CONTACT FOR ENGAGING SAID CONDUCTOR COMPRISING AN ELECTRICALLY CONDUCTIVE DISC MEMBER HAVING A PLURALITY OF TEETH SPACED AROUND ITS PERIPHERY, THE SPACING BEING SUCH THAT AT LEAST ONE OF SAID TEETH MESHES BETWEEN ADJACENT TURNS OF THE CONDUCTOR. 